Conventional orthogonal frequency division multiplexing (OFDM) networks and orthogonal frequency division multiple access (OFDMA) network improve channel reliability by spreading and/or coding data traffic and control signals over multiple subcarriers (i.e., tones). Different user devices (i.e., subscriber stations, mobile stations, etc.) are allocated different sets of subcarriers (or tones) for transmitting and receiving the data and control signals. The subcarrier frequencies are orthogonal to each other, thereby minimizing interference between user devices.
OFDM techniques are particularly advantageous in multiple-input, multiple output (MIMO) wireless networks that employ multiple antennas (i.e., Smart antennas) to transmit OFDM signals to the user devices. However, in conventional MIMO techniques based on OFDM transmission, the same subband (or set of subcarriers) is allocated to a given user device from each of the transmit antennas. However, due to independent fading from each of the transmit antennas, the user device may experience different channel quality for each of the transmit antennas even for the same subband. Therefore, transmitting on the same subband from each antenna for each user devices results in less than optimal performance, because the subband selected for transmission may not be good on each of the transmit antennas.
Additionally, conventional MIMO techniques allocate subcarrier to subscriber stations using inefficient control signaling schemes that create a large amount of overhead. In some conventional systems, a subscriber station is allocated to a set of subcarriers using a control message containing a bitmap. The bitmap contains a Logic 1 or 0 for each subcarrier in the frequency band. Thus, a subscriber station is allocated to 64 subcarriers out of a total of 512 subcarriers by means of a 512-bit bitmap containing 64 Logic 1 values at the allocated subcarriers.
Alternatively, groups of subcarriers (i.e., subbands) may be allocated together. Thus, for example, the 512 subcarriers may be divided into 16 groups (or subbands), each subband (SB) containing 32 subcarriers (contiguous or non-contiguous). These subbands may be allocated by a control message containing a 32-bit bitmap. In still another embodiment, the control message may allocate logically sequential subbands using a start-end indicator that identifies the first and the last of the sequential subbands.
Therefore, there is a need in the art for improved apparatuses and methods for transmitting OFDM signals from a multi-antenna OFDM transmitter to an OFDM receiver. In particular, there is a need for a multi-antenna OFDM base station that uses an efficient control messaging scheme for allocating multiple user devices (e.g., subscriber stations) to selected subbands or subcarrier groups.